There is a method which has been prevailingly applied to magnetic recording materials, wherein thermoplastic resins, such as vinyl chloride-vinyl acetate resins, vinyl chloride-vinylidene chloride resins, cellulose resins, acetal resins, urethane resins, acrylonitrile-butadiene resins and so on, are employed as their respective binders independently or in combination of two or more thereof, as described in U.S. Pat. Nos. 4,068,040, 3,242,005, 4,352,859, 3,840,400, 4,115,290, 3,681,137, 3,781,210, 4,049,566, 4,336,308, 4,049,871, 4,361,627 and 3,262,813. However, such a method has the defect that sufficient abrasion resistance cannot be imparted to a magnetic layer and thereby, the running course of magnetic tapes is stained.
Further, there have been known a method of utilizing thermosetting resins such as melamine resins, urea resins and the like, and a method of using composition prepared by adding to the aforementioned thermoplastic resins such binders as to undergo chemically a cross-linking reaction, e.g., isocyanate compounds, epoxy compounds or the like. However, these methods suffer from (1) the defect that use of cross-linking binders causes lowering of storage stability of the resinous solution in which a magnetic substance is dispersed, that is, shortening of the pot life of the magnetic coating, which makes it impossible to maintain uniformity of the physical properties of the magnetic coating and in its turn the homogeniety of the magnetic tape, and (2) after coating and drying, a heat processing step is required to harden the coat and therefore, it takes a lot of time to produce finished goods.
With the invention of obviating the above-described defects, methods of producing magnetic recording materials in which acrylic acid ester oligomers and monomers are employed as a binder and wherein the setting thereof is carried out by irradiation with electron beams are disclosed in Japanese Patent Publication No. 12423/'72; Japanese Patent Application (OPI) Nos. 15104/'72, 77433/'75, 86130/'82 and 127926/'82; U.S. Pat. No. 3,871,908; and British Pat. Nos. 2,055,877-B and 2,084,589. However, such methods have failed in ensuring a high state of electromagnetic properties and a high degree of durability to magnetic recording materials.
Recently, high state of electromagnetic properties have been required in particular for magnetic recording materials. Consequently, improvement in the dispersibility of ferromagnetic fine powders has become a more pressing need. Poor dispersibility of ferromagnetic fine powders causes lowering of output, and is responsible for noise.